Reference to Related Application
The present invention is disclosed (but not claimed) in a co-pending Application, Ser. No. 972,111, filed Dec. 21, 1978, in the name of R. G. Heitz and entitled "EFFICIENT, HIGH POWER BATTERY MODULE; D.C. TRANSFORMERS AND MULTI-TERMINAL D.C. POWER NETWORKS UTILIZING SAME".
Alkali metal/chalcogen battery cells are subject to a type of failure (irreversible open circuit failure) which has not been commonly experienced with mechanically intact batteries of other types. If just one of a series string of such cells fails on open circuit, the circuit through the entire string is broken.
One of the most important contemplated uses for alkali metal/chalcogen cells--storage of off-peak power--involves charging (and discharge) voltages of up to several hundred kilovolts. To accommodate such voltages, with cells which have no-load potentials of about 2 volts, requires the use of series strings of large numbers of cells. For example, a battery comprising a string of 9100 functioning cells will be required for a nominal working potential of about 18,000 volts.
If a by-pass means is not provided for cells which fail on open circuit, the entire string will be out of service until the bad cell is replaced. This necessitates either a high standby labor cost (for replacing cells as soon as they go bad) or a high capital cost for enough extra batteries to carry the system between periodic maintenance operations.
If a by-pass means is provided, enough extra cells can be included in each string so that the string will not have to be turned off for maintenance until, say, 10% of its component cells have failed. Thus, in an installation for 18,000 volts a string of 10,000 cells, each with an automatic by-pass, would allow 900 cells to fail and be by-passed before the string voltage would drop far enough to require maintenance.
Alkali metal/chalcogen cells, as exemplified by sodium/sulfur cells, can fail on open circuit in several ways. One of these ways is peculiar to the now well known type of cell in which the electrolyte/separator takes the form of a large number of cation-conductive, hair-like hollow fibers, filled with liquid sodium (for example) and immersed in a molten sodium sulfide/sulfur mixture (for example). The sodium which migrates (as Na.sup.+ ions) through the fiber walls during discharge is replaced from a reservoir containing sodium in contact with an anodic current collecting means. If the sodium level in the reservoir drops too far, as on overdischarge of the cell or if a fiber failure results in leakage of the sodium out of the reservoir, contact between the sodium and the anodic electron collecting means will be essentially broken and the circuit through the cell develops a high resistance. An effectively irreversible open circuit "failure" has then occurred.
Irreversible open circuit failure can also occur by corrosion of the cathodic current collector, resulting in formation of high resistance surface coatings or even in substantial dissolution of the collector material in the catholyte (at abnormally high temperatures).
The former type of open circuit failure will not occur in types of alkali metal/chalcogen cells in which the hollow fibers are replaced by a single large tube (or by several smaller tubes) into which it is feasible to extend the anodic electron collecting means. However, the latter type of open circuit failure can occur in any such cell in which the cathodic current collecting means (which may consist of or include the catholyte container) is not practically immune to attack by the catholyte at the most elevated temperatures the cell may experience.
Another type of open circuit failure can occur in sodium/sulfur cells of any type if the sodium to sulfur ratio is high enough so that the cell can be overdischarged until the proportion of the sodium ions in the catholyte is sufficient to cause it to solidify. Although it may be possible to reliquify the catholyte by heating the cell to a higher temperature, the failure is --in effect --irreversible at the normal working temperature of the cell. Also, if the cell is of the hollow fiber type, the fibers are likely to be damaged when the catholyte solidifies.
The desirability of a simple by-pass means which can be used one-to-one with individual high temperature battery cells has been recognized. A brief study on the use of copper-oxide electric cut-outs for this purpose was cursorily reported on in a July-September 1976 Progress Report on "High Performance Batteries for Off-Peak Energy Storage and Electrical Vehicle Propulsion", under the heading "Automatic Repair of a Battery Cell With an Open Circuit", by F. Hornstra, E. C. Berrill and S. Faist, Argonne National Laboratory, Argonne, Illinois. The report concluded that further study would be necessary before the use of such cut-outs could be applied successfully to high temperature batteries.
Incorporation in a sodium/sulfur cell of a circuit breaking device responsive to temperature or current is disclosed in U.S. Pat. No. 4,011,366. However, this device does not function as a by-pass and the disclosure of the patent is limited to cells connected in parallel.
A solid-state battery protection system, by means of which each cell in a battery is monitored and any cell which exhibits voltage outside of pre-selected limits is by-passed (and taken out of the circuit), is disclosed in NASA Technical Brief, LEW--12039; Fall 1976; NTN-77/0584. However, this system does not appear to be suitable for high temperature applications.
Accordingly, the prior art does not appear to have provided a workable means for by-passing alkali metal/chalcogen cells which have failed on open circuit.